Devices and methods for heat bending glass sheets are well known in the art. For example, see U.S. Pat. Nos. 5,383,990; 6,318,125; 6,158,247; and 5,443,669.
FIG. 1 is a schematic diagram illustrating a conventional apparatus and method for heat bending glass sheets in making a laminated product such as a vehicle windshield. Vehicle windshields are typically curved, and thus require first and second curved (as a result of heat bending) glass sheets laminated to one another via a polymer interlayer. First glass substrate 1 has a multi-layer solar control coating 3 thereon (e.g., low-E coating 3 including at least one IR reflecting layer of a material such as Ag); while second glass substrate 5 need not be coated.
Referring to FIG. 1, two flat glass substrates 1, 5 are placed in a bending furnace (e.g., on a bending mold) in an overlapping manner by interposing an optional lubricating powder (not shown) such as sodium hydrogen carbonate, cerite, magnesium oxide, silica, or the like between contacting surfaces of the two glass substrates. The glass substrates 1, 5 are then heated using infrared (IR) emitting heating elements 7 to a processing temperature(s) near a softening point of the glass (e.g., from about 550 to 850 degrees C., more preferably from about 580 to 750 degrees C.) in order to soften the overlapping glass substrates 1, 5. Upon softening, the glass substrates 1, 5 (including any solar control coating 3 thereon) are bent by their deadweight (i.e., sagging) along a shaping surface of a bending mold (not shown) into the desired curved shape appropriate for the vehicle windshield being made. A press bending apparatus (not shown) may optionally be used after the glass is sufficiently softened (the press bending may be conducted as the final step before cooling the glass).
After being heat bent in such a manner, the bent glass substrates 1, 5 (with solar control coating 3 still on substrate 1) are separated from one another and a polymer inclusive interlayer sheet (e.g., PVB) is interposed therebetween. The glass substrates 1, 5 are then laminated to one another via the polymer inclusive interlayer 9 in order to form the resulting vehicle windshield shown in FIG. 2.
Different vehicle windshield models require different shapes. Some shapes require more extensive bending (i.e., tighter/smaller radii of curvature after bending) than others. As windshields requiring extensive/extreme bending are becoming more popular, the need for high performance solar control coatings (e.g., including one or more IR reflecting Ag layers) has also increased. An example high performance solar control coating 3 is disclosed in WO 02/04375 (and thus counterpart U.S. Ser. No. 09/794,224, now U.S. Pat. No. 6,576,349, filed Feb. 28, 2001), both hereby incorporated herein by reference.
Unfortunately, it has been found that when using conventional glass bending techniques, certain solar control coatings cannot on a regular basis withstand the bending process(es) sometimes used without suffering damage, especially when very tight radius of curvature(s) are required for the windshield. For example, when using conventional IR heating techniques to bend glass sheets for a windshield when the aforesaid solar control coating is provided on one of the sheets, the tightest radius of curvature consistently achievable in the windshield without coating damage is about 3,500 mm. However, windshields having a smaller (or more extreme) tightest radius of curvature are sometimes required, and thus cannot be consistently made using IR heating techniques without damaging the aforesaid coating. Set forth below is an explanation as to why certain solar control coatings have a hard time withstanding conventional IR heat bending processes without suffering undesirable damage.
Referring to FIG. 1, conventional glass bending heating elements emit IR radiation 8 in the near, mid and far IR ranges. By this we mean that heating elements 7 emit each of near-IR (700–4,000 nm; or 0.7 to 4.0 μm), mid-IR (4,000–8,000 nm; or 4–8 μm), and far-IR radiation. Much of the IR radiation that reaches the glass to be bent is in the near-IR range, as the peak of this IR radiation is often in the near-IR range. In certain example instances, at least about 50% of the IR radiation that reaches the glass to be bent is in the near-IR range, sometimes 70% or higher.
As shown in FIG. 3, it has been found that typical soda lime silica glass (often used for substrates 1, 5) absorbs much incident IR radiation at wavelengths above about 3–4 μm (microns). FIG. 3 shows that soda lime silica glass is substantially opaque to IR radiation above 3–4 μm, but rather transmissive of IR radiation below 3–4 μm. Unfortunately, this means that a significant amount of IR radiation in the near-IR range (from 0.7 to 3–4 μm) is not absorbed by the glass substrate(s) 1 and/or 5 and as a result passes therethrough and reaches solar control coating 3. As used herein, the phrase “from 0.7 to 3–4 μm” means from 0.7 m to 3 and/or 4 μm.
Unfortunately, certain of this near-IR radiation which is not absorbed by the glass substrate and reaches solar control coating 3, is absorbed by the coating 3 (e.g., by Ag layer(s) of the coating) thereby causing the coating 3 to heat up. This problem (significant heating of the coating) is compounded by: (a) certain solar control coatings 3 have a room temperature absorption peak (e.g., of 20–30% or more) at wavelengths of approximately 1 μm in the near IR range, at which wavelengths the underlying glass is substantially transmissive, and (b) the absorption of many solar control coatings 3 increases with a rise in temperature thereof (e.g., sheet resistance Rs of Ag layer(s) increase along with rises in temperature). In view of (a) and (b) above, it can be seen that the peak absorption of certain solar control coatings 3 at near-IR wavelengths of about 1 μm can increase from the 20–30% range to the 40–60% range or higher when the coating temperature increases from room temperature to 500 degrees C. or higher, thereby enabling the coating to heat up very quickly when exposed to significant amounts of near-IR radiation.
Coating 3 is more susceptible to being damaged when it is unnecessarily heated up during the glass bending process. When coating 3 is damaged, the bent glass substrate 1 with the damaged coating thereon is typically discarded and cannot be commercially used.
This problem (i.e., coating overheating) affects the shapes that can be attained in the bending process. In particular, more extreme glass bending requires more extreme heating (i.e., at higher temperatures and/or for longer times) of the glass to be bent. As mentioned above, when the coating 3 of WO 02/04375 (and thus counterpart U.S. Ser. No. 09/794,224, now U.S. Pat. No. 6,576,349) is provided on one of the glass substrates (1 or 5), the tightest radius of curvature of bent glass consistently achievable using conventional IR radiation to bend without damaging the coating is about 3,500 mm. This is, or course, undesirable if a portion of a windshield is desired to have a tighter (smaller) radius of curvature; i.e., to be more bent.
U.S. Pat. No. 5,827,345 discloses the use of microwave energy during the bending and tempering of glass. However, the '345 patent uses microwave energy solely for its heating speed. Thus, the '345 patent fails to recognize or solve the problem(s) (e.g., coating over-heating and/or need for smaller radii of curvature(s) for bent glass) addressed and solved by the instant invention. In other words, the '345 patent fails to disclose or suggest using microwave radiation to heat coated glass sheet(s) for the purpose of enabling the coated glass sheet(s) to be bent to a more significant degree or smaller tightest radius of curvature.
An object of this invention is to minimize the peak temperature attained (and/or the time at which a peak temperature is attained) by a solar control coating 3 during a process for heat bending a glass substrate that supports the coating.
Another object of this invention is to provide an apparatus and/or method for heat bending glass substrate(s)/sheet(s), the apparatus and/or method being designed to reduce the amount of near-IR radiation that reaches the glass substrate(s) to be bent during the bending process.
Another object of this invention is to provide a method and/or apparatus for heat bending coated glass substrate(s), where the substrate(s) is/are heated using at least microwave radiation. The use of microwave radiation (and/or less IR radiation) to heat the glass enables the solar control coating supported by the glass substrate to reach a lesser temperature and/or a maximum temperature for a lesser time period than if only conventional IR radiation was used for heating.
By reducing the maximum coating temperature and/or the time at which the coating realizes this temperature, certain embodiments of this invention can achieve one or more of the following advantages: (a) the solar control coating is less likely to be damaged during the bending process of an underlying glass substrate, (b) higher degrees of bending (i.e., tighter radii of curvature) of an underlying glass substrate(s) can be achieved without damaging the solar control coating; and/or (c) power consumption of the heater may possibly be reduced in certain instances.
Another object of certain example embodiments of this invention is to use microwave radiation to heat a coated glass substrate(s) during a bending process, in order to enable the coated substrate to be bent so as to have a tightest radius of curvature of no greater than about 3,000 mm, more preferably no greater than about 2,000 mm, and even more preferably no greater than about 1,000 mm without significant coating damage. In certain example instances, the tightest radius of curvature for a windshield may be as low as 200–500 mm.
Another object of this invention is to fulfill one or more of the above-listed objects.
In certain example embodiments of this invention, one or more of the above-listed objects is/are fulfilled by providing a method of making a vehicle windshield, the method comprising: directing microwave radiation toward first and second glass substrates in order to heat the glass substrates for bending, and wherein a coating supported by one of the glass substrates comprises at least one infrared (IR) reflecting layer comprising silver (Ag) sandwiched between at least a pair of dielectric layers; and bending the glass substrates and laminating the substrates together to form a vehicle windshield so that a tightest radius of curvature defined by a major surface of the resulting windshield is no greater than about 3,000 mm.
In other embodiments of this invention, one or more of the above-listed objects is/are fulfilled by providing a method of bending coated glass, the method comprising: directing microwave radiation toward a glass substrate in order to heat the glass substrate for bending, and wherein a coating supported by the glass substrate comprises at least one infrared (IR) reflecting layer comprising silver (Ag) sandwiched between at least a pair of dielectric layers; and bending the glass substrate and the coating thereon so that a tightest radius of curvature defined by a major surface of the resulting bent glass substrate is no greater than about 3,000 mm.